1. Field of the Invention
The present invention generally relates to an improved flotation apparatus and process for purifying a waste effluent by forming a separable, buoyant floc out of the waste matter suspended in the effluent. The invention utilizes a novel mixing and floc dispersion means to achieve an improved flotation apparatus and process.
2. Description of the Prior Art
The purification of waste effluents by flotation devices is well known in the prior art. In general, such devices purify effluents such as waste water by forming a buoyant floc out of the particles suspended in the water. The buoyant floc rises to the surface of the water, where it is skimmed off by an arrangement of skimming paddles.
More specifically, the stream of effluent to be treated is first mixed with one or more conditioning agents and then conducted into a flotation tank, where the effluent and flocculating agent is mixed with a stream or cloud of small bubbles of buoyant gas. The conditioning agent may either be a precipitating agent, a coagulating agent, a flocculating agent, or may function to change the pH of the effluent. In any event, the conditioning agent causes the particles of waste matter suspended in the effluent to flocculate into agglomerates of ever-increasing size. Since this flocculating process is carried out in the presence of the stream or cloud of tiny, buoyant bubbles produced in the flotation tank, some of these bubbles become entrapped in the growing agglomerates of flock. The entrapped bubbles of buoyant gas accordingly render the agglomerates of floc buoyant, and cause them to float up to the surface of the flotation tank. The skimming mechanism rakes the buoyant floc over to a floc accumulation region of the flotation tank, where the skimming paddles ultimately rake the floc out of the tank. The treated effluent is then discharged from the treatment tank.
Ideally, a flotation device should be capable of effectively purifying a large volume of effluent in as short a time and as small a volume as possible in order to minimize the expenses associated with both the construction and the space requirements of the device.
To this end, the flotation device should have a mixing means capable of quickly and effectively admixing the streams of effluent, conditioning agent and bubbles to form a buoyant floc out of the waste particles suspended in the effluent. Moreover, the flotation device should have a means for uniformly distributing the resulting buoyant floc over transverse axis of the floc accumulation area of the treatment tank so that the floc may be easily and effectively skimmed off the fluid surface. Further, the flotation device should have a means for dampening fluid currents in the tank which could agitate the bubbles out of the floc floating on the surface of the fluid, thereby causing it to sink to the tank bottom. Such floc sinking currents tend to arise when large streams of effluent are introduced into a relatively small tank. The resulting sunken floc not only thwarts the effectiveness of the skim removal means, but often necessitates either the addition of a tank floor-sweeping mechanism, or frequent use of troublesome and time-consuming tank maintenance procedures. Finally, it would be desirable if the flotation device accomplished all of the aforementioned functions with a means which required little or no maintenance.
Unfortunately, the prior art does not disclose a flotation apparatus having a means which achieves all of the aforementioned ideal criteria. For example, while there is some recognition in U.S. Pat. No. 3,679,056 of the desirability of efficiently admixing the stream of effluent with a stream of buoyant gas bubbles, the mixing chamber of this device includes a gas capturing lip which can impede the flow of buoyant floc from the bottom to the top of the tank. Such an obstruction could shake the buoyant gas bubbles out of the flock, causing it to sink to the bottom. An additional drawback in the design is the lack of any apparent means for uniformly distributing the buoyant gas bubbles across the transverse axis of the tank. Hence, this design could generate a non-uniform layer of floc across the surface of the effluent in the tank, with buoyant floc collecting more densely at the middle of the tank than the sides of the tank. Additionally, this design has no apparent means to dampen floc-sinking currents which could be generated from the inflow of fluid from the feed inlet. The combination of these apparent deficiencies may account for the inclusion of a sludge raking means in this design.
U.S. Pat. Nos. 3,418,236 and 3,446,488 also disclose flotation devices for purifying a stream of effluent. However, like the previously discussed U.S. Pat. No. 3,679,056, neither of the devices disclosed in these patents has any apparent means to effectively dampen floc-sinking currents caused by the inflow of effluent. Additionally, while U.S. Pat. No. 3,418,236 discusses the desirability of admixing the stream of buoyant gas bubbles with the effluent being treated, both of these patents disclose the use of a mixing body or baffle to achieve these ends. The inventor has found, in the course of empirical experimentation, that such mixing baffles tend to accumulate deposits of agglomerating floc. If such mixing baffles are not regularly cleaned, the surface of the mixing baffles becomes irregular, generating localized turbulences which not only result in a non-uniform dispersion of floc, but which also agitate the buoyant bubbles out of some of the floc passing over the surface of the mixing baffle, causing it to sink and accumulate into the bottom of the tank.
The flotation apparatus disclosed in U.S. Pat. No. 3,929,640 also discloses a design which falls short of fulfilling the aforementioned ideal criteria. The invention in the U.S. Pat. No. 3,929,640 discloses a mixing and flow mechanism which includes a horizontal, disk-like baffle. As stated before, such baffles tend to accumulate deposits of floc, which in turn adversely affects the formation of a uniform layer of buoyant floc on the surface of the tank, while generating non-buoyant floc which accumulates on the bottom of the treatment tank.
Clearly, all of the foregoing prior art designs include features such as mixing baffles which cause them to fall short of fulfilling all of the aforementioned ideal criteria of a simple and compact flotation apparatus capable of effectively purifying a large volume of effluent in a short time.